Digital printing techniques have been developed that allow a printing system to receive instructions directly from a computer without the need to prepare printing plates. Amongst these are color laser printers that use the xerographic process. Color laser printers using dry toners are suitable for certain applications, but they do not produce images of a photographic quality acceptable for publications such as magazines.
A process that is better suited for short run high quality digital printing is used in the HP-Indigo printer. In this process, an electrostatic image is produced on an electrically charged image bearing cylinder by exposure to laser light. The electrostatic charge attracts oil-based inks to form a color ink image on the image bearing cylinder. The ink image is then transferred by way of a blanket cylinder onto paper or any other printing medium, the substrate.
Inkjet and bubble jet processes are commonly used in home and office printers. In these processes droplets of ink are sprayed onto a final substrate in an image pattern. In general, the resolution of such processes is limited due to wicking by the inks into paper substrates, unless coated paper is used. However, using substrates with special coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below its surface is a costly option that is unsuitable for certain printing applications, especially for commercial printing. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly and time consuming steps are needed to dry the ink, so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible. Inkjet printing directly onto a substrate results in poor image quality also because of variation of the distance between the print head and the surface of the substrate.
Indirect or offset digital printing systems have been disclosed in the patent literature that comprise an intermediate transfer member, an image forming system operative to form an ink image on the surface of the intermediate transfer member, apparatus serving to dry the ink image as it is transported by the transfer member, and an impression station at which the dried ink image is transferred from the intermediate transfer member onto a substrate supported by an impression cylinder that is rotatable about a fixed axis and driven independently of the intermediate transfer member.
Using an offset printing system overcomes many problems associated with inkjet printing directly onto the substrate. For example, it allows the distance between the surface of the intermediate transfer member and the inkjet print head to be maintained constant and it reduces wetting of the substrate as the ink can be dried on the image transfer surface before being applied to the substrate. Consequently, the final image quality on the substrate is less affected by the physical properties of the substrate.